A. Field of the Invention
This invention relates generally to an intrinsically safe DC-to-DC power supply having a voltage-clamping Zener diode barrier, which is suitable for use in an overfill protection device. The invention relates more particularly to a novel design for such a power supply that dynamically self-compensates for variations in breakdown voltage of the Zener diode barrier (e.g., due to component tolerances and temperature drift and aging effects) so as to maximize the supply's output power while maintaining a nominal biasing current flowing through the Zener barrier.
B. Description of the Related Art
Tanks used for storing or transporting flammable fluids such as gasoline, diesel fuel and other hazardous petroleum products are often equipped with overfill protection devices to stop the filling operation when the tanks are full, and thereby prevent waste and protect the environment from spillage due to overfilling.
The tanks can be mounted on tanker trucks or located, e.g., underground at service stations. Tanker trucks are typically filled with the fluids using pumping equipment at loading racks of marketing terminals, and underground storage tanks are typically gravity filled from the trucks. An overfill protection device is used with each tank to disable the pumping equipment at the marketing terminal or to close a truck-mounted flow valve at the service station when the limit of the tank's capacity is reached.
The overfill protection device typically has a detection circuit and a disable circuit. The detection circuit has a probe located within the tank which generates a sensor signal that indicates when the fluid within the tank exceeds a pre-determined level. The detection circuit also has a microprocessor-containing controller mounted near the tank that controls the operation of the probe. The detection circuit provides the sensor signal to the disable circuit over a suitable electrical cable. In response to the sensor signal indicating that the tank is full, the disable circuit operates to stop flow into the tank (e.g., depending on where the tank is located, by disabling the pumping equipment at the loading rack or by closing the flow valve on the truck).
The detection circuit also includes a power supply for energizing the probe and the controller. Because the fluids are flammable, the overflow protection device is typically and preferably designed to be "intrinsically safe." As defined in applicable standards promulgated by governmental agencies and industrial organizations, an intrinsically safe circuit cannot produce any spark or thermal effect, either during "normal" or under any likely "fault" condition, which is capable of causing the ignition of a mixture of the flammable fluid and its vapor or other combustible material in air. As a practical matter, this means that the power supplies used in overfill protection devices have special components, e.g., fuses, voltage-clamping Zener-diode barriers, and current-limiting output resistors, that limit the current and voltage delivered to circuitry located within fire-hazard locations.
More specifically, a conventional intrinsically-safe ("I.S.") power supply for use in an overflow protection device has a conventional voltage regulator for delivering a regulated, substantially constant-magnitude voltage to the detection circuit.
For precision regulation, the conventional voltage regulator often includes a feedback arrangement to maintain the substantially constant output voltage despite variations in the magnitude of the input voltages applied from a power source to the regulator. A voltage divider in the feedback circuit establishes the magnitude of the output voltage. The voltage divider typically includes a variable resistor or potentiometer connected between an output terminal and a control terminal of the voltage regulator, and a second resistor connected between its control terminal and a return path back to the power source, i.e., ground.
The power supply also has specific I.S. components, including a current-limiting output resistor connected between a regulator output terminal and the power supply output terminal to limit the output current, a voltage-clamp that is also connected to the regulator output terminal to limit the output voltage to a safe level, and a fuse.
The voltage-clamp is typically formed as a Zener barrier, which includes at least one power Zener diode connected between the regulator output terminal and ground. For intrinsically-safe power supply operation, each Zener diode typically has a second Zener diode in parallel with it for redundancy in case of component failure.
The fuse is connected in the voltage input circuit for the regulator to limit the maximum current drawn by the power supply. By limiting that current, the fuse also limits the maximum power dissipation in the other I.S. components, i.e., the output resistor and the voltage clamp.
These I.S. components insure intrinsically safe operation of the power supply even during "fault" conditions. Such fault conditions for which the circuit is designed, include an overvoltage on the input terminal of the regulator, in response to which the Zener barrier limits the supply's output voltage to below a preselected limit by shunting excessive current due to the overvoltage to ground. Another possible fault condition is a short circuit across the supply's output, in response to which the Zener barrier limits the resulting voltage, and, together with the output resistor, limits the resulting current.
While such conventional I.S. power supplies are generally suitable for their intended applications, they do have drawbacks. For precision voltage regulation in overfill protection devices, the power supplies are typically individually factory calibrated (e.g., by adjusting the potentiometer of the voltage divider) before shipment to compensate for variances in component tolerances particularly in the breakdown voltages of the Zener diodes of the voltage clamp. Because the power supplies are expected to operate for years outdoors over wide variations in ambient temperature, the factory calibration also is intended to compensate for anticipated temperature-dependent and aging effects on the breakdown voltages of those Zener diodes.
Factory calibration requires an additional time-consuming step during post-assembly testing of the circuit. Moreover, the factory calibration requires that compromises be made in the current operating point of the Zener diodes of the voltage clamp. To prevent the Zener diodes from conducting excessive current in normal operation due to changes in the Zener breakdown voltage, the regulator output voltage is typically set during calibration to a lower than optimal level. Accordingly, conventional I.S. power supplies achieve less than optimal efficiency in terms of the magnitude of the output power from the power supply for a given power input.
It would be desirable to provide an intrinsically safe power supply that does not require such calibration adjustments, and that operates more efficiently than the prior art circuit.